Avoidance of bouncing and splashing in droplet-based fluid transport

ABSTRACT

A system for fluid transport is provided where a quantity of fluid is held in a reservoir. A droplet generator is employed to generate droplets from the fluid, for example a nozzle-based system or a nozzleless system such as an acoustic ejection system. A generated droplet has a trajectory whereby it arrives at a target. A circuit is used to modify one or more characteristics of the generated droplet in a way which increases the likelihood that the droplet will not splash or bounce when it arrives at the target. The circuit may in different embodiments control the speed of the droplet or the Weber number of the droplet. The circuit may create an electric field in an area of space where the droplet passes. The circuit may charge the droplet by causing it to contact ions.

TECHNICAL FIELD

This application is a continuation of U.S. patent application Ser. No.12/944,364, filed Nov. 11, 2010, which is a continuation of U.S. patentapplication Ser. No. 11/080,024, filed Mar. 14, 2005, both of which areincorporated by reference herein for all purposes.

BACKGROUND

There exists a need in pharmaceutical, biotechnological, medical, andother industries to be able to quickly screen, identify, analyze, and/orprocess large numbers or varieties of fluids. As a result, muchattention has been focused on developing efficient, precise, andaccurate fluid handling methods. For example, automated robotic systemshave been used in combination with precise registration technologies todispense reagents through automated pick-and-place (“suck-and-spit”)fluid handling systems. Similarly, some efforts have been directed toadapting printing technologies, particularly inkjet printingtechnologies, to form biomolecular arrays. For example, U.S. Pat. No.6,015,880 to Baldeschwieler et al. is directed to array preparationusing multistep in situ synthesis. Such synthesis may involve usinginkjet technology to dispense reagent-containing droplets to a locus ona surface chemically prepared to permit covalent attachment of thereagent.

There are tradeoffs in the choice of a fluid transport system. Forexample, most fluid handling systems presently in use require thatcontact be established between the fluid to be transferred and anassociated solid surface on the transferring device. Such contacttypically results in surface wetting that causes unavoidable fluidwaste, a notable drawback when the fluid to be transferred is rareand/or expensive. When fluid transport systems are constructed usingnetworks of tubing or other fluid transporting conduits, air bubbles canbe entrapped or particulates may become lodged in the networks. Nozzlesof ordinary inkjet printheads are also subject to clogging, especiallywhen used to eject a macromolecule-containing fluid at elevatedtemperatures, a situation commonly associated with such technologies. Asa result, ordinary fluid transport technologies may produce improperlysized or misdirected droplets.

A number of patents have described the use of focused acoustic radiationto dispense fluids such as inks and reagents. For example, U.S. Pat. No.4,308,547 to Lovelady et al. describes a liquid drop emitter thatutilizes acoustic principles to eject droplets from a body of liquidonto a moving document to result in the formation of characters orbarcodes thereon. A nozzleless inkjet printing apparatus is used suchthat controlled drops of ink are propelled by an acoustical forceproduced by a curved transducer at or below the surface of the ink.Similarly, U.S. Patent Application Publication No. 20020037579 to Ellsonet al. describes a device for acoustically ejecting a plurality of fluiddroplets toward discrete sites on a substrate surface for depositionthereon. U.S. Patent Application Publication No. 20020094582 to Williamsdescribes technologies that employ focused acoustic technology as well.In contrast to inkjet printing devices, focused acoustic radiation maybe used to effect nozzleless fluid ejection, and devices using focusedacoustic radiation are not generally subject to clogging and thedisadvantages associated therewith, e.g., misdirected fluid orimproperly sized droplets.

Since fluids used in pharmaceutical, biotechnological, and otherscientific industries may be rare and/or expensive, techniques capableof handling small volumes of fluids provide readily apparent advantagesover those requiring relatively larger volumes. Typically, fluids foruse in combinatorial methods are provided as a collection or library oforganic and/or biological compounds. In many instances, well plates areused to store a large number of fluids for screening and/or processing.Well plates are typically of single piece construction and comprise aplurality of identical wells, wherein each well is adapted to contain asmall volume of fluid. Such well plates are commercially available instandardized sizes and may contain, for example, 96, 384, 1536, or 3456wells per well plate.

Transport of fluid droplets may be directed at an existing volume offluid. For example, in any fluid transport system that employs discretedroplets, it may be desirable to use a number of smaller droplets totransport the fluid rather than a single larger droplet. Each dropletafter the first will potentially impact an existing volume of fluid.

When a fluid droplet is directed at an existing volume of fluid, it isoften desirable that the droplet coalesce with the existing volume.Instead of coalescing, a droplet may bounce or splash, which is oftenundesirable. Bouncing and splashing may also be undesirable when thedroplets are directed at a solid target. For example, the target may bea well plate in which the droplet is supposed to be placed entirely inan identified individual well in accordance with the transfer protocolbeing employed, whereas splashing might cause a portion of the fluid inthe droplet to fall into a different well instead.

Bouncing of droplets when they encounter a solid or an existing volumeof fluid has been observed for many years. It is believed that thephenomenon involves not simply the droplet and the solid or volume offluid but also a cushion of air between the droplet and the solid orvolume. Precise predictions of when droplet bouncing and splashing willoccur based on conventional fluid parameters such as viscosity andsurface tension are often not within the capabilities of computationalfluid mechanics, so that empirical investigation is a preferred methodof analyzing questions which relate to droplet bouncing and splashing. Asummary of certain empirical investigations is found in M. Orme,“Experiments on Droplet Collisions, Bounce, Coalescence and Disruption,”Progress in Energy and Combustion Science, vol. 23, pp. 65-79, 1997,which contains a number of references to the literature.

SUMMARY OF THE INVENTION

The invention is in general a system for fluid transport. A quantity offluid is held in a reservoir. A droplet generator is employed togenerate droplets from the fluid, for example a nozzle-based system or anozzleless system such as an acoustic ejection system. The dropletgenerator may or may not make contact with the fluid in order togenerate the droplet. The droplet generator may be set up to move fromone reservoir to another in order to be able to eject from more than onereservoir. In a common arrangement, a number of reservoirs form part ofan integral structure, e.g., a well plate, and the structure is moved bysuitable mechanical or electromechanical systems, potentially undercomputer control, into a suitable position with respect to the dropletgenerator.

The droplet generator is controlled by a controller, preferablyelectronic and connectable to computers through some communicationssystem. The droplets after generation arrive at a target, which may befor example a well plate, a porous or non-porous surface, a substrate,or a structure which is to be coated by the droplets. A circuit controlsone or more characteristics of the generated droplet, increasing thelikelihood that the droplet will not splash or bounce on arriving at thetarget. This circuit may be controlled by the controller or may formpart of the controller. The target may already have a quantity of fluid,and the droplet may coalesce with that quantity of fluid.

In certain preferred embodiments, the circuit for controlling one ormore characteristics may control the speed of the droplet as it arrivesat the target. The speed may be controlled in preferred ranges. Inanother embodiment, the circuit for controlling may control the Webernumber of the droplet. In doing so, the circuit may control for examplethe diameter of the droplet or its velocity or both. In anotherpreferred embodiment, the circuit may provide an electric field in anarea of space through which the droplet passes. This electric field mayresult in polarization or charging of the droplet. In another preferredembodiment, the circuit may charge the droplet by causing it to contactions.

The invention also encompasses methods for the transport of fluids. Inthese methods, a droplet is generated from a quantity of fluid in areservoir. A trajectory of the droplet is controlled in such a way thatit arrives at a target. One or more characteristics of the generateddroplet are controlled so as to increase the likelihood that the dropletwill not splash or bounce on arriving at the target.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in detail below with reference to thefollowing drawings, wherein like reference numerals indicate acorresponding structure throughout the several views.

FIGS. 1A and 1B, collectively referred to as FIG. 1, schematicallyillustrate in simplified cross-sectional view the operation of a focusedacoustic ejection device in the preparation of a plurality of featureson a substrate surface. FIG. 1A shows the acoustic ejector acousticallycoupled to a first reservoir and having been activated in order to ejecta first droplet of fluid from within the reservoir toward a particularsite on a substrate surface. FIG. 1B shows the acoustic ejectoracoustically coupled to a second reservoir and having been activated toeject a second droplet of fluid from within the second reservoir.

FIG. 2 illustrates in cross-sectional schematic view the ejection ofdroplets of fluid from a volume of fluid on a substrate surface into aninlet opening disposed on a terminus of a capillary.

FIG. 3 depicts the approach of a droplet to a target with an electricfield in the direction of droplet approach.

FIG. 4 depicts an exemplary circuit of the invention.

FIGS. 5A-5C show the speed control achievable through control of theacoustic energy in an acoustic ejection system.

FIGS. 6A-6C show the effect of droplet speed on the likelihood ofdroplet coalescence for 70% DMSO/30% water.

DETAILED DESCRIPTION OF THE INVENTION

Before describing the present invention in detail, it is to beunderstood that this invention is not limited to specific fluids,biomolecules, or device structures, as such may vary. It is also to beunderstood that the terminology used herein is for the purpose ofdescribing particular embodiments only, and is not intended to belimiting.

It must be noted that, as used in this specification and the appendedclaims, the singular forms “a,” “an,” and “the” include both singularand plural referents unless the context clearly dictates otherwise.Thus, for example, reference to “a reservoir” includes a plurality ofreservoirs as well as a single reservoir, reference to “a droplet”includes a plurality of droplets as well as single droplet, and thelike.

In describing and claiming the present invention, the followingterminology will be used in accordance with the definitions set outbelow.

The terms “acoustic coupling” and “acoustically coupled” as used hereinrefer to a state wherein an object is placed in direct or indirectcontact with another object so as to allow acoustic radiation to betransferred between the objects without substantial loss of acousticenergy. When two items are indirectly acoustically coupled, an “acousticcoupling medium” is needed to provide an intermediary through whichacoustic radiation may be transmitted. Thus, an ejector may beacoustically coupled to a fluid, e.g., by immersing the ejector in thefluid or by interposing an acoustic coupling medium between the ejectorand the fluid, in order to transfer acoustic radiation generated by theejector through the acoustic coupling medium and into the fluid.

The term “array” as used herein refers to a two-dimensional arrangementof features, such as an arrangement of reservoirs (e.g., wells in a wellplate) or an arrangement of different moieties, including ionic,metallic, or covalent crystalline, e.g., molecular crystalline,composite, ceramic, vitreous, amorphous, fluidic, or molecular materialson a substrate surface (as in an oligonucleotide or peptidic array).Arrays are generally comprised of regular features that are ordered, asin, for example, a rectilinear grid, parallel stripes, spirals, and thelike, but non-ordered arrays may be advantageously used as well. Inparticular, the term “rectilinear array” as used herein refers to anarray that has rows and columns of features wherein the rows and columnstypically, but not necessarily, intersect each other at a ninety-degreeangle. An array is distinguished from the more general term “pattern” inthat patterns do not necessarily contain regular and ordered features.Arrays typically but do not necessarily comprise at least about 4 toabout 10,000,000 features, generally in the range of about 4 to about1,000,000 features.

The terms “biomolecule” and “biological molecule” are usedinterchangeably herein to refer to any organic molecule that is, was, orcan be a part of a living organism, regardless of whether the moleculeis naturally occurring, recombinantly produced, or chemicallysynthesized in whole or in part. The terms encompass, for example,nucleotides, amino acids, and monosaccharides, as well as oligomeric andpolymeric species, such as oligonucleotides and polynucleotides;peptidic molecules, such as oligopeptides, polypeptides, and proteins;saccharides, such as disaccharides, oligosaccharides, polysaccharides,and mucopolysaccharides or peptidoglycans (peptido-polysaccharides); andthe like. The terms also encompass ribosomes, enzyme cofactors,pharmacologically active agents, and the like. Additional informationrelating to the term “biomolecule” can be found in U.S. PatentApplication Publication No. 20020037579 to Ellson et al.

The term “capillary” is used herein to refer to a conduit having a boreof small dimension. Typically, capillaries for electrophoresis that arefree standing tubes have an inner diameter in the range of about 50 toabout 250 μm. Capillaries with extremely small bores integrated to otherdevices, such as openings for loading microchannels of microfluidicdevices, can be as small as 1 μm, but in general these capillaryopenings are in the range of about 10 to about 100 μm. In the context ofdelivery to a mass analyzer in electrospray-type mass spectrometry, theinner diameter of capillaries may range from about 0.1 to about 3 mm andpreferably from about 0.5 to about 1 mm. In some instances, a capillarycan represent a portion of a microfluidic device. In such instances, thecapillary may be an integral or affixed (permanently or detachably)portion of the microfluidic device.

The term “fluid” as used herein refers to matter that is nonsolid, or atleast partially gaseous and/or liquid, but not entirely gaseous. A fluidmay contain a solid that is minimally, partially, or fully solvated,dispersed, or suspended. Examples of fluids include, without limitation,aqueous liquids (including water per se and salt water) and nonaqueousliquids such as organic solvents and the like. As used herein, the term“fluid” is not synonymous with the term “ink” in that an ink mustcontain a colorant and may not be gaseous.

The terms “focusing means” and “acoustic focusing means” refer to ameans for causing acoustic waves to converge at a focal point, either bya device separate from the acoustic energy source that acts like anoptical lens, or by the spatial arrangement of acoustic energy sourcesto effect convergence of acoustic energy at a focal point byconstructive and destructive interference. A focusing means may be assimple as a solid member having a curved surface, or it may includecomplex structures such as those found in Fresnel lenses, which employdiffraction in order to direct acoustic radiation. Suitable focusingmeans also include phased array methods as are known in the art anddescribed, for example, in U.S. Pat. No. 5,798,779 to Nakayasu et al.and by Amemiya et al. (1997) Proceedings of the 1997 IS&T NIP13International Conference on Digital Printing Technologies, pp. 698-702.Additional information regarding acoustic focusing is provided in U.S.patent application Ser. No. 10/066,546, entitled “Acoustic SampleIntroduction for Analysis and/or Processing,” filed Jan. 30, 2002,inventors Ellson and Mutz.

The terms “library” and “combinatorial library” are used interchangeablyherein to refer to a plurality of chemical or biological moietiesarranged in a pattern or an array such that the moieties areindividually addressable. In some instances, the plurality of chemicalor biological moieties is present on the surface of a substrate, and inother instances the plurality of moieties represents the contents of aplurality of reservoirs. Preferably, but not necessarily, each moiety isdifferent from each of the other moieties. The moieties may be, forexample, peptidic molecules and/or oligonucleotides.

The “limiting dimension” of an opening refers herein to the theoreticalmaximum diameter of a sphere that can pass through an opening withoutdeformation. For example, the limiting dimension of a circular openingis the diameter of the opening. As another example, the limitingdimension of a rectangular opening is the length of the shorter side ofthe rectangular opening. The opening may be present on any solid bodyincluding, but not limited to, sample vessels, substrates, capillaries,microfluidic devices, and ionization chambers. Depending on the purposeof the opening, the opening may represent an inlet and/or an outlet.

The term “moiety” refers to any particular composition of matter, e.g.,a molecular fragment, an intact molecule (including a monomericmolecule, an oligomeric molecule, or a polymer), or a mixture ofmaterials (for example, an alloy or a laminate).

The term “near,” when used to refer to the distance from the focal pointof the focused acoustic radiation to the surface of the fluid from whicha droplet is to be ejected, indicates that the distance should be suchthat the focused acoustic radiation directed into the fluid results indroplet ejection from the fluid surface; one of ordinary skill in theart will be able to select an appropriate distance for any given fluidusing straightforward and routine experimentation. Generally, however, asuitable distance between the focal point of the acoustic radiation andthe fluid surface is in the range of about 1 to about 15 times thewavelength of the speed of sound in the fluid, more typically in therange of about 1 to about 10 times that wavelength, preferably in therange of about 1 to about 5 times that wavelength.

“Optional” or “optionally” means that the subsequently describedcircumstance may or may not occur, so that the description includesinstances where the circumstance occurs and instances where it does not.

The term “radiation” is used in its ordinary sense and refers toemission and propagation of energy in the form of a waveform disturbancetraveling through a medium such that energy is transferred from oneparticle of the medium to another, generally without causing anypermanent displacement of the medium itself. Thus, radiation may refer,for example, to electromagnetic waveforms as well as acousticvibrations.

Accordingly, the terms “acoustic radiation” and “acoustic energy” areused interchangeably herein and refer to the emission and propagation ofenergy in the form of sound waves. As with other waveforms, acousticradiation may be focused using a focusing means, as discussed below.Although acoustic radiation may have a single frequency and associatedwavelength, acoustic radiation may take a form, e.g. a linear chirp,that includes a plurality of frequencies. Thus, the term “characteristicwavelength” is used to describe the mean wavelength of acousticradiation having a plurality of frequencies.

The term “reservoir” as used herein refers to a receptacle or chamberfor containing a fluid. In some instances, a fluid contained in areservoir will have a free surface, e.g., a surface that allows acousticradiation to be reflected therefrom or a surface from which a dropletmay be acoustically ejected. A reservoir may also be a locus on asubstrate surface within which a fluid is constrained.

The term “substrate” as used herein refers to any material having asurface onto which one or more fluids may be deposited. The substratemay be constructed in any of a number of forms including, for example,wafers, slides, well plates, or membranes. In addition, the substratemay be porous or nonporous as required for deposition of a particularfluid. Suitable substrate materials include, but are not limited to,supports that are typically used for solid phase chemical synthesis,such as polymeric materials (e.g., polystyrene, polyvinyl acetate,polyvinyl chloride, polyvinyl pyrrolidone, polyacrylonitrile,polyacrylamide, polymethyl methacrylate, polytetrafluoroethylene,polyethylene, polypropylene, polyvinylidene fluoride, polycarbonate, anddivinylbenzene styrene-based polymers), agarose (e.g., Sepharose®),dextran (e.g., Sephadex®), cellulosic polymers and otherpolysaccharides, silica and silica-based materials, glass (particularlycontrolled pore glass, or “CPG”) and functionalized glasses, ceramics,and such substrates treated with surface coatings, e.g., withmicroporous polymers (particularly cellulosic polymers such asnitrocellulose), microporous metallic compounds (particularlymicroporous aluminum), antibody-binding proteins (available from PierceChemical Co., Rockford Ill.), bisphenol A polycarbonate, or the like.Additional information relating to the term “substrate” can be found inU.S. Patent Application Publication No. 200200377579 to Ellson et al.

The term “substantially” as in, for example, the phrase “substantiallydeviate from a predetermined volume,” refers to a volume that does notdeviate by more than about 25%, preferably 10%, more preferably 5%, andmost preferably at most 2%, from the predetermined volume. Other uses ofthe term “substantially” involve an analogous definition.

The term “sample vessel” as used herein refers to any hollow or concavereceptacle having a structure that allows for sample processing and/oranalysis. Thus, a sample vessel has an inlet opening through whichsample may be introduced and an optional, but preferred, outlet openingthrough which processed or analyzed sample may exit.

The invention may be employed with any type of fluid dispenser thatserves to dispense one or more droplets of fluid from a reservoir. Anyfluid droplet dispensing techniques known in the art may be used inconjunction with the present invention. For example, the invention maybe used with dispensers such as inkjet printheads (both thermal andpiezoelectric), pipettes, capillaries, syringes, displacement pumps,rotary pumps, peristaltic pumps, vacuum devices, flexible or rigidtubing, valves, manifolds, pressurized gas canisters, and combinationsthereof. While nonacoustic techniques may be used to dispense fluid fromthe reservoir, the invention is particularly suited for use with nozzleless acoustic ejection techniques that employ focused acoustic radiationgenerated by acoustic ejectors, such as those described in U.S. PatentApplication Publication No. 20020037579 to Ellson et al. Thispublication sets forth that an ejector may be acoustically coupled to areservoir containing a fluid in order to eject a droplet therefrom. Insome instances, the reservoir may be a well of a well plate. Since thisdevice configuration allows droplets to be ejected from near the base ofa well, uncontrolled electrostatic charge anywhere in the well, e.g.,the base or sidewalls, may have a strong effect influence on the volumeand/or trajectory of such droplets.

Since acoustic ejection provides a number of advantages over other fluiddispensing technologies, some embodiments of the invention employ adevice for acoustically ejecting a droplet of fluid from a reservoir.The device is comprised of a reservoir adapted to contain a fluid, anejector for ejecting a droplet from the reservoir, and a means forpositioning the ejector in acoustic coupling relationship to thereservoir. The ejector comprises an acoustic radiation generator forgenerating acoustic radiation and a focusing means for focusing theacoustic radiation generated by the generator. As described in U.S.Patent Application Publication No. 20020037579 to Ellson et al., theacoustic radiation is focused at a focal point within and sufficientlynear the fluid surface in the reservoir to result in the ejection ofdroplets therefrom. Furthermore, a means is provided for reducing anyuncontrolled electrostatic charge on the device or a portion thereofthat alters the volume and/or trajectory of a droplet ejected from thereservoir. As a result, the volume and/or trajectory of the ejecteddroplet do not substantially deviate from a predetermined volume and/orpredetermined trajectory.

The device may be constructed to include the reservoir as an integratedor permanently attached component of the device. However, to providemodularity and interchangeability of components, it is preferred thatthe device be constructed with a removable reservoir. Optionally, aplurality of reservoirs many be provided. Generally, the reservoirs arearranged in a pattern or an array to provide each reservoir withindividual systematic addressability. In addition, while each of thereservoirs may be provided as a discrete or stand-alone item, incircumstances that require a large number of reservoirs, it is preferredthat the reservoirs be attached to each other or represent integratedportions of a single reservoir unit. For example, the reservoirs mayrepresent individual wells in a well plate.

Many well plates suitable for use with the device are commerciallyavailable and may contain, for example, 96, 384, 1536, or 3456 wells perwell plate, having a full skirt, half skirt, or no skirt. The wells ofsuch well plates typically form rectilinear arrays. Manufacturers ofsuitable well plates for use in the employed device include Corning,Inc. (Corning, N.Y.) and Greiner America, Inc. (Lake Mary, Fla.).However, the availability of such commercially available well platesdoes not preclude the manufacture and use of custom-made well platescontaining at least about 10,000 wells, or as many as 100,000 to 500,000wells, or more. The wells of such custom-made well plates may formrectilinear or other types of arrays. As well plates have becomecommonly used laboratory items, the Society for Biomolecular Screening(Danbury, Conn.) has formed the Microplate Standards DevelopmentCommittee to recommend and maintain standards to facilitate theautomated processing of small volume well plates on behalf of and foracceptance by the American National Standards Institute.

Furthermore, the material used in the construction of reservoirs must becompatible with the fluids contained therein. Thus, if it is intendedthat the reservoirs or wells contain an organic solvent such asacetonitrile, polymers that dissolve or swell in acetonitrile would beunsuitable for use in forming the reservoirs or well plates. Similarly,reservoirs or wells intended to contain DMSO must be compatible withDMSO. For water-based fluids, a number of materials are suitable for theconstruction of reservoirs and include, but are not limited to, ceramicssuch as silicon oxide and aluminum oxide, metals such as stainless steeland platinum, and polymers such as polyester andpolytetrafluoroethylene. For fluids that are photosensitive, thereservoirs may be constructed from an optically opaque material that hassufficient acoustic transparency for substantially unimpairedfunctioning of the device. Thus, the reservoir may be adapted to containany type of fluid, metallic or nonmetallic, organic or inorganic.

When a plurality of reservoirs is employed, the acoustic radiationgenerator may have to be aligned with each reservoir during operation,discussed infra. In order to reduce the amount of movement and timeneeded to align the generator successively with each reservoir, it ispreferable that the center of each reservoir be located not more thanabout 1 centimeter, more preferably not more than about 1.5 millimeters,still more preferably not more than about 1 millimeter and optimally notmore than about 0.5 millimeter, from a neighboring reservoir center.These dimensions tend to limit the size of the reservoirs to a maximumvolume. The reservoirs are constructed to contain typically no more thanabout 1 mL, preferably no more than about 100 μL, more preferably nomore than about 10 μL, still more preferably no more than about 1 μL,and optimally no more than about 1 nL, of fluid. The reservoirs may beeither completely or partially filled with fluid. For example, fluid mayoccupy a volume of about 10 pL to about 100 nL.

When an array of reservoirs is provided, each reservoir may beindividually, efficiently, and systematically addressed. Although anytype of array may be employed, arrays comprised of parallel rows ofevenly spaced reservoirs are preferred. Typically, though notnecessarily, each row contains the same number of reservoirs. Optimally,rectilinear arrays comprising X rows and Y columns of reservoirs areemployed with the invention, wherein X and Y are each at least 2. Insome instances, X may be greater than, equal to, or less than Y. Inaddition, nonrectilinear arrays as well as other geometries may beemployed. For example, hexagonal, spiral, or other types of arrays maybe used. In some instances, the invention may be employed with irregularpatterns of reservoirs, e.g., droplets randomly located on a flatsubstrate surface. In addition, the invention may be used withreservoirs associated with microfluidic devices.

Moreover, the invention may be used to transport fluids of virtually anytype and amount desired. The fluid may be aqueous and/or nonaqueous.Examples of fluids include, but are not limited to, aqueous fluidsincluding water per se and water-solvated ionic and non-ionic solutions;organic solvents; lipidic liquids; suspensions of immiscible fluids; andsuspensions or slurries of solids in liquids. Because the invention isreadily adapted for use with high temperatures, fluids such as liquidmetals, ceramic materials, and glasses may be used, as described in U.S.Patent Application Publication No. 20020140118. In some instances, thereservoir may contain a biomolecule, nucleotidic, peptidic, orotherwise. In addition, the invention may be used in conjunction withdispensers for dispensing droplets of immiscible fluids, as described inU.S. Patent Application Publication Nos. 2002037375 and 20020155231, orto dispense droplets containing pharmaceutical agents, as discussed inU.S. Patent Application Publication No. 20020142049 and U.S. patentapplication Ser. No. 10/244,128, entitled “Precipitation of SolidParticles from Droplets Formed Using Focused Acoustic Energy,” filed,Sep. 13, 2002, inventors Lee, Ellson and Williams.

Any of a variety of focusing means may be employed to focus acousticradiation so as to eject droplets from a reservoir. For example, one ormore curved surfaces may be used to direct acoustic radiation to a focalpoint near a fluid surface. One such technique is described in U.S. Pat.No. 4,308,547 to Lovelady et al. Focusing means with a curved surfacehave been incorporated into the construction of commercially availableacoustic transducers such as those manufactured by Panametrics Inc.(Waltham, Mass.). In addition, Fresnel lenses are known in the art fordirecting acoustic energy at a predetermined focal distance from anobject plane. See, e.g., U.S. Pat. No. 5,041,849 to Quate et al. Fresnellenses may have a radial phase profile that diffracts a substantialportion of acoustic energy into a predetermined diffraction order atdiffraction angles that vary radially with respect to the lens. Thediffraction angles should be selected to focus the acoustic energywithin the diffraction order on a desired object plane. It should benoted that acoustic focusing means exhibiting a variety of F-numbers maybe employed with the invention. As discussed in U.S. Pat. No. 6,416,164to Stearns et al., however, low F-number focusing places restrictions onthe reservoir and fluid level geometry and provides relatively limiteddepth of focus, increasing the sensitivity to the fluid level in thereservoir. Thus, the focusing means suitable for use with the inventiontypically exhibits an F-number of at least about 1. Preferably, thefocusing means exhibits an F-number of at least about 2.

There are a number of ways to acoustically couple the ejector to areservoir and thus to the fluid therein. One such approach is throughdirect contact, as is described, for example, in U.S. Pat. No. 4,308,547to Lovelady et al., wherein a focusing means constructed from ahemispherical crystal having segmented electrodes is submerged in aliquid to be ejected. The aforementioned patent further discloses thatthe focusing means may be positioned at or below the surface of theliquid. However, this approach for acoustically coupling the focusingmeans to a fluid is undesirable when the ejector is used to ejectdifferent fluids in a plurality of containers or reservoirs, as repeatedcleaning of the focusing means would be required in order to avoidcross-contamination. The cleaning process would necessarily lengthen thetransition time between each droplet ejection event. In addition, insuch a method, fluid would adhere to the ejector as it is removed fromeach container, wasting material that may be costly or rare.

Thus, a preferred approach is to acoustically couple the ejector to thereservoir without contacting any portion of the ejector, e.g., thefocusing means, with the fluids to be ejected. When a plurality ofreservoirs is employed, a positioning means is provided for positioningthe ejector in controlled and repeatable acoustic coupling with each ofthe fluids in the reservoirs to eject droplets therefrom withoutsubmerging the ejector therein. This typically involves direct orindirect contact between the ejector and the external surface of eachreservoir. When direct contact is used in order to acoustically couplethe ejector to each reservoir, it is preferred that the direct contactbe wholly conformal to ensure efficient acoustic energy transfer. Thatis, the ejector and the reservoir should have corresponding surfacesadapted for mating contact. Thus, if acoustic coupling is achievedbetween the ejector and reservoir through the focusing means, it isdesirable for the reservoir to have an outside surface that correspondsto the surface profile of the focusing means. Without conformal contact,efficiency and accuracy of acoustic energy transfer may be compromised.In addition, since many focusing means have a curved surface, the directcontact approach may necessitate the use of reservoirs having aspecially formed inverse surface.

When an ejector is placed in indirect contact with a reservoir, anacoustic coupling medium may be interposed between the reservoir andejector. Typically, the acoustic coupling medium is a fluid. Inaddition, the acoustic coupling medium is preferably an acousticallyhomogeneous material that is substantially free of material havingdifferent acoustic properties than the fluid medium itself. Furthermore,it is preferred that the acoustic coupling medium be comprised of amaterial having acoustic properties that facilitate the transmission ofacoustic radiation without significant attenuation in acoustic pressureand intensity. Also, the acoustic impedance of the coupling mediumshould facilitate the transfer of energy from the coupling medium intothe reservoir. An aqueous fluid, such as water per se, may be employedas an acoustic coupling medium. Ionic additives, e.g., salts, maysometimes be added to the coupling medium to increase the conductivityof the coupling medium.

A single ejector is preferred, although an acoustic ejection system mayinclude a plurality of ejectors. When a single ejector is employed, themeans for positioning the ejector may be adapted to provide relativemotion between the ejector and reservoirs. The positioning means shouldallow for the ejector to move from one reservoir to another quickly andin a controlled manner, thereby allowing fast and controlled scanning ofthe reservoirs to effect droplet ejection therefrom. Thus, various meansfor positioning the ejector in acoustic coupling relationship to thereservoir are generally known in the art and may involve, e.g., devicesthat provide movement having one, two, three, four, five, six, or moredegrees of freedom. Accordingly, when rows of reservoirs are provided,relative motion between the acoustic radiation generator and thereservoirs may result in displacement of the acoustic radiationgenerator in a direction along the rows. Similarly, when a rectilineararray of reservoirs is provided, the ejector may be movable in arow-wise direction and/or in a direction perpendicular to both the rowsand columns.

Current positioning technology allows for the ejector positioning meansto move from one reservoir to another quickly and in a controlledmanner, thereby allowing fast and controlled ejection of different fluidsamples. That is, current commercially available technology allows theejector to be moved from one reservoir to another, with repeatable andcontrolled acoustic coupling at each reservoir, in less than about 0.1second for high performance positioning means and in less than about 1second for ordinary positioning means. A custom designed system willallow the ejector to be moved from one reservoir to another withrepeatable and controlled acoustic coupling in less than about 0.001second.

Acoustic ejection also enables rapid ejection of droplets from one ormore reservoirs, e.g., at a rate of at least about 1,000,000 dropletsper minute from the same reservoir, and at a rate of at least about100,000 drops per minute from different reservoirs, assuming that thedroplet size does not exceed about 10 μm in diameter. One of ordinaryskill in the art will recognize that the droplet generation rate is afunction of drop size, viscosity, surface tension, and other fluidproperties. In general, the droplet generation rate increases withdecreasing droplet diameter, and 1,000,000 droplets per minute isachievable for most aqueous fluid drops under about 10 μm in diameter.

Acoustic ejection may be used in any context where precise placement ofa fluid droplet is desirable or necessary. In particular, the inventionmay be employed to improve accuracy and precision associated withnozzleless acoustic ejection. For example, it is described in U.S.Patent Application Publication No. 20020037579 to Ellson et al. thatacoustic ejection technology may be used to form biomolecular arrays.Similarly, acoustic ejection technology may be employed to format aplurality of fluids, e.g., to transfer fluids from odd-sized bulkcontainers to wells of a standardized well plate or to transfer fluidsfrom one well plate to another. Furthermore, as described in U.S. PatentApplication Publication Nos. 20020109084 and 20020125424, each to Ellsonet al., focused acoustic radiation may serve to eject a droplet of fluidfrom a reservoir into any sample vessel for processing and/or analyzinga sample molecule, e.g., into a sample introduction interface of a massspectrometer, an inlet opening that provides access to the interiorregion of a capillary, or an inlet port of a microfluidic device.Similarly, the invention may be used to transport droplets ofanalysis-enhancing fluid on a sample surface in order to prepare thesample for analysis, e.g., for MALDI or SELDI-type analysis.

In order to prepare an array on a substrate surface, the substrate mustbe placed in droplet-receiving relationship to a reservoir. Thus, theinvention may also employ a positioning means for positioning thesubstrate. With respect to the substrate positioning means and theejector positioning means, it is important to keep in mind that thereare two basic kinds of motion: pulse and continuous. For the ejectorpositioning means, pulse motion involves the discrete steps of moving anejector into position, emitting acoustic energy, and moving the ejectorto the next position; again, using a high performance positioning meanswith such a method allows repeatable and controlled acoustic coupling ateach reservoir in less than 0.1 second. A continuous motion design, onthe other hand, moves the ejector and the reservoirs continuously,although not necessarily at the same speed, and provides for ejectionduring movement. Since the pulse width is very short, this type ofprocess enables over 10 Hz reservoir transitions, and even over 1000 Hzreservoir transitions. Similar engineering considerations are applicableto the substrate positioning means.

From the above, it is evident that the relative positions and spatialorientations of the various components may be altered depending on theparticular desired task at hand. In such a case, the various componentsof the device may require individual control or synchronization todirect droplets onto designated sites on a substrate surface. Forexample, the ejector positioning means may be adapted to eject dropletsfrom each reservoir in a predetermined sequence associated with an arrayof designated sites on the substrate surface. Any positioning means ofthe present invention may be constructed from, e.g., levers, pulleys,gears, a combination thereof, or other mechanical means known to one ofordinary skill in the art.

FIG. 1 illustrates an exemplary focused acoustic ejection devicesuitable for use with the invention, in simplified cross-sectional view.As with all figures referenced herein, in which like parts arereferenced by like numerals, FIG. 1 is not to scale, and certaindimensions may be exaggerated for clarity of presentation. The device 11includes a plurality of reservoirs, i.e., at least two reservoirs—afirst reservoir indicated at 13 and a second reservoir indicated at 15.Each reservoir contains a combination of two or more immiscible fluids,and the individual fluids as well as the fluid combinations in thedifferent reservoirs may be the same or different. As shown, reservoir13 contains fluid 14, and reservoir 15 contains fluid 16. Fluids 14 and16 have fluid surfaces respectively indicated at 17 and 19. As shown,the reservoirs are of substantially identical construction so as to besubstantially acoustically indistinguishable, but identical constructionis not a requirement. The reservoirs are shown as separate removablecomponents but may, if desired, be fixed within a plate or othersubstrate. Each of the reservoirs 13 and 15 is axially, symmetric asshown, having vertical walls 21 and 23 extending upward from circularreservoir bases 25 and 27 and terminating at openings 29 and 31,respectively, although other reservoir shapes may be used. The materialand thickness of each reservoir base should be such that acousticradiation may be transmitted therethrough and into the fluid containedwithin the reservoirs.

The device also includes an acoustic ejector 33 comprised of an acousticradiation generator 35 for generating acoustic radiation, and a focusingmeans 37 for focusing the acoustic radiation at a focal point near thefluid surface from which a droplet is to be ejected, wherein the focalpoint is selected so as to result in droplet ejection. The focal pointmay be in the upper fluid layer or the lower fluid layer, but ispreferably just below the interface therebetween. As shown in FIG. 1,the focusing means 37 may comprise a single solid piece having a concavesurface 39 for focusing acoustic radiation, but the focusing means maybe constructed in other ways as discussed below. The acoustic ejector 33is thus adapted to generate and focus acoustic radiation so as to ejecta droplet of fluid from each of the fluid surfaces 17 and 19 whenacoustically coupled to reservoirs 13 and 15, respectively. The acousticradiation generator 35 and the focusing means 37 may function as asingle unit controlled by a single controller, or they may beindependently controlled, depending on the desired performance of thedevice. Typically, single ejector designs are preferred over multipleejector designs, because accuracy of droplet placement, as well asconsistency in droplet size and velocity, are more easily achieved witha single ejector.

In operation, each reservoir 13 and 15 of the device is filled withdifferent fluids, as explained above. The acoustic ejector 33 ispositionable by means of ejector positioning means 43, shown belowreservoir 13, in order to achieve acoustic coupling between the ejectorand the reservoir through acoustic coupling medium 41. If dropletejection onto a substrate is desired, a substrate 49 may be positionedabove and in proximity to the first reservoir 13 such that one surfaceof the substrate, shown in FIG. 1 as underside surface 51, faces thereservoir and is substantially parallel to the surface 17 of the fluid14 therein. The substrate 49 is held by substrate positioning means 53,which, as shown, is grounded. Thus, when the substrate 49 is comprisedof a conductive material, the substrate 49 is grounded as well. Once theejector, the reservoir, and the substrate are in proper alignment, theacoustic radiation generator 35 is activated to produce acousticradiation that is directed by the focusing means 37 to a focal point 55near the fluid surface 17 of the first reservoir. As a result, droplet57 is ejected from the fluid surface 17, optionally onto a particularsite (typically although not necessarily, a pre-selected, or“predetermined” site) on the underside surface 49 of the substrate. Theejected droplet may be retained on the substrate surface by solidifyingthereon after contact; in such an embodiment, it is necessary tomaintain the substrate surface at a low temperature, i.e., at atemperature that results in droplet solidification after contact.Alternatively, or in addition, a molecular moiety within the dropletattaches to the substrate surface after contact, through adsorption,physical immobilization, or covalent binding.

Then, as shown in FIG. 18, a substrate positioning means 53 may be usedto reposition the substrate 49 (if used) over reservoir 15 in order toreceive a droplet therefrom at a second site. FIG. 1B also shows thatthe ejector 33 has been repositioned by the ejector positioning means 59below reservoir 15 and in acoustically coupled relationship thereto byvirtue of acoustic coupling medium 41. Once properly aligned, as shownin FIG. 1B, the acoustic radiation generator 35 of ejector 33 isactivated to produce acoustic radiation that is then directed byfocusing means 37 to a focal point within the reservoir fluids inreservoir 15, thereby ejecting droplet 63, optionally onto thesubstrate.

It should be evident that such operation is illustrative of how anacoustic ejector may be used to eject a plurality of droplets fromreservoirs in order to form a pattern, e.g., an array, on the substratesurface 51. It should be similarly evident that an acoustic ejector maybe adapted to eject a plurality of droplets from one or more reservoirsonto the same site of the substrate surface. Furthermore, the ejectionof a plurality of droplets may involve one or more ejectors. In someinstances, the droplets are ejected successively from one or morereservoirs. In other instances, droplets are ejected simultaneously fromdifferent reservoirs.

As depicted in FIG. 2, the invention may be used with a single reservoirto transport fluid into an inlet opening of a target vessel. Axiallysymmetric capillary 49 having an inlet opening 50 disposed on a terminus51 thereof is provided as a target vessel. Due to the axial symmetry ofthe capillary 49, the inlet opening 50 has a circular cross section. Assuch, the opening has a limiting dimension equal to its diameter.

A hemispherical volume of fluid 14 on a substantially flat surface 25 ofa substrate 13 serves a reservoir. The shape of fluid 14 is a functionof the sample wetting properties with respect to the substrate surface25. Thus, the shape can be modified with any of a number of surfacemodification techniques. In addition, an ejector 33 is providedcomprising an acoustic radiation generator 35 for generating radiation,and a focusing means 37 for directing the radiation at a focal pointnear the surface 17 of the fluid 14. The ejector 33 is shown in acousticcoupling relationship to the substrate 13 through coupling fluid 41.Proper control of acoustic wavelength and amplitude results in theejection of a droplet 57 from the fluid 14 on the substrate 13. As thedroplet 57 is shown having a diameter only slightly smaller than thediameter of the inlet opening 49, it is evident that this configurationrequires strict control over the droplet size and trajectory.

The invention is in general a system or method for fluid transport. Aquantity of fluid is held in a reservoir. A droplet generator isemployed to generate droplets from the fluid, for example a nozzle-basedsystem or a nozzleless system such as an acoustic ejection system. Thedroplet generator may or may not make contact with the fluid in order togenerate the droplet. The droplet generator may be set up to move fromone reservoir to another in order to be able to eject from more than onereservoir. In a common arrangement, a number of reservoirs form part ofan integral structure, e.g., a well plate, and the structure is moved bysuitable mechanical or electromechanical systems, potentially undercomputer control, into a suitable position with respect to the dropletgenerator.

The droplet generator is controlled by a controller, preferablyelectronic and connectable to computers through some communicationssystem. The droplets after generation arrive at a target, which may befor example a well plate, a porous or non-porous surface, a substrate,or a structure which is to be coated by the droplets. The invention alsoincludes a circuit which controls a characteristic of the generateddroplet, increasing the likelihood that the droplet will not splash orbounce on arriving at the target. This circuit may be controlled by thecontroller or may form part of the controller. The target may alreadyhave a quantity of fluid, and the droplet may coalesce with thatquantity of fluid.

In general the invention may be used with a range of fluids and targets.For example, a fluid in the target with which coalescence could bedesired to occur may be of different composition from the fluid in thedroplet. A common compositional difference encountered in practice isratio of DMSO to water where solutions comprising mixtures of DMSO andwater are transported. Compositional differences may lead to differencesin physical parameters such as viscosity and dielectric constant. Thus,for example, a 70% DMSO/30% water mixture has a viscosity of roughly 3.5centipoise while an aqueous buffer would have a viscosity of roughly 1centipoise. Thus the droplets of the invention may have a viscositywhich is more than 3 times that of the fluid at the target, or viceversa.

The fluid with which coalescence could be desired to occur may be ofapproximately the same volume as the droplet, for example anotherdroplet of similar size or two droplets of similar size, or it may be alarger quantity of fluid, for example a quantity of fluid more than 100times larger than the droplet.

A preferred controller contains a microprocessor with suitable memoryand software or firmware. The microprocessor may be running an operatingsystem specialized for real time and embedded applications, as forexample QNX from QNX Software Systems (Ottawa, Ontario, Canada). Somecontrollers may contain two or more microprocessors and distributefunctions among them as is convenient from the point of view of thedesign and operation of the system. Such a controller is preferablyconnected to a display allowing direct operator interaction with thefluid transport system by pressing of buttons or through a touch screen.Such a controller is also preferably provided with communicationssoftware, firmware and/or hardware which allows it to communicate withcomputers including general-purpose computers which can form part of anoverall laboratory or manufacturing automation network, and which wouldallow the controller to operate the fluid transport system in anautomated manner based on commands or information received from otherentities in the overall laboratory or manufacturing automation system.

An overall laboratory automation system may include, for example, acarousel for holding well plates, a robot arm for moving well platesfrom one instrument to another, a variety of analytical instruments andreaction chambers, a pin based fluid transfer system, and/or an acousticejection system. The overall purposes of the system may include takingquantities of fluids and subjecting them to analyses (including forexample the ascertainment of their composition and physical properties),reactions designed to produce particular moieties, and purificationsteps, all the while potentially keeping track, by computerized or othermeans, of the origin and destination of each fluid in the system and ofthe processes and results for each fluid. The system may also beemployed to generate for further use objects which contain or are coatedwith fluids moved by the system.

The tracking of the origin, destination, processes, and results for eachfluid may be performed, for example, by having controllers such as thefluid transport system controller communicate that information to ageneral purpose computer which stores the information as flat files orin a database. Fluids are conveniently identified by assigning anidentifier to each well plate in the system and by tracking what is doneto each well in each plate at particular times in a way that allows oneto produce an overall history for the contents of each well of eachplate. It must be kept in mind in this regard that not all changes influids in the system take place as a result of deliberate or plannedaction; some may be inevitable changes that occur as a result of thepassage of time, as for example the absorption of water from thesurrounding air or the evaporation of fluids in storage.

In a laboratory automation system it will generally be necessary tointegrate equipment from different manufacturers. In this connection theadherence to particular standards may be a desirable feature of a fluidtransport system. Certain fluid transport systems which form part of amanufacturing environment may be required to meet further standardsrelating to manufacturing as well as being able to support overallsystem conformance with the norms of “Good Manufacturing Practice” (GMP)as understood by the pharmaceutical industry. In particular applicationsthe fluids being transported may be pathogenic requiring specialmeasures for their handling which may impact on the design of thecontroller.

A preferred controller of a fluid transport system will also optionallycontain detailed information for achieving efficient transport of fluidwithin the system. Such information might be, for example, preferredranges of droplet speeds as discussed below, preferred ranges of Webernumbers, and physical characteristics of the fluids being manipulated,as for example their conductivity and permittivity, which are alsodiscussed below, and also their density, viscosity, surface tension, andthe like. Such information will also include algorithms for operating ina suitably time-sequenced manner the different actuators of the fluidtransport system, for example well plate transport systems, pin basedfluid transport systems, robot arms, acoustic ejector transducers, andthe electrodes discussed in connection with certain embodiments below.The time-sequenced algorithms may be stored in the form of tables whichare interpreted by suitable software or firmware or they may be codeddirectly as programs or they may be a mixture of tables and code. Thetime-sequenced algorithms may take into account information obtainedfrom various sensors in the system, for example position and temperaturesensors and digital cameras, as well as stored or measuredcharacteristics of the fluids being manipulated. Preferably thealgorithms may be modified in the field, e.g., by software downloadsover a network, to take into account the most recent knowledge about thebest known methods for the operation of the fluid transport system. Thecontroller may also have learning capabilities in which it can, throughanalysis of fluids, determine by itself some of the parameters mostsuitable for their efficient transport.

One way to control characteristics of the droplets in a fluid transportsystem to reduce the likelihood of splashing or bounce is to control thespeed with which the droplet approaches the target. The manner in whichthe speed control is accomplished may depend on the droplet generationtechnique.

Gravity and friction against the surrounding air or other gas have animpact on droplet speed which has been studied in connection with themodeling of the behavior of raindrops. Some control of the impact ofthese forces is possible, particularly in the case of downward flowwhere the drop height can influence the speed of the droplet at impact.Because it is difficult to zero out the effect of these forces, theireffect may need to be taken into account in designing systems even wherethe primary control of droplet speed is achieved by other means. Thesystem of the invention encompasses and contemplates the possibilitythat the droplet will travel, upon being generated, in a directiongenerally against the force of the earth's gravitational attraction, inparticular in a direction 90 to 180 degrees with respect to the vectorwhich gives the direction of the earth's gravitational attraction.

Nanoliter scale droplets of water-like properties have been studied. Forsuch droplets it is found that speeds which are preferable for dropletapproach with coalescence are in the range of 0.2 to 10 m/s. A morepreferable range is 1.0 to 2.5 m/s, and within that range a value above1.5 m/s is particularly preferred.

Electric fields may have an impact on the speed of a droplet, asdiscussed for example in U.S. Pat. No. 5,541,627 to Quate, which statesgenerally that water is attracted to an electric field. This impact mayexist even if the droplet is uncharged if the electric field causes thedroplet to become electrically polarized, as will occur if the dropletconsists primarily of a polarizable solvent such as water.

It is believed that an uncharged drop will only feel a net force on itin a nonuniform electric field. In any field, the drop will develop aninduced dipole charge, i.e. a separation of charges, just as is the casefor any dielectric. A dipole in turn will be attracted toward aconcentration of electric field, that is toward a region of greaterfield. In a uniform field, the dipole experiences no net force.Mathematically, the force is proportional to the gradient of the squareof the external electric field.

When an acoustic ejection system is used for droplet generation, it isfound that the speed of the droplet is controllable within a rangethrough the amount of acoustic energy used to cause the ejection tooccur. It has been found, for example, that for 5 nL droplets of 70%DMSO/30% water in an acoustic ejection system using an F2 lens, controlof the speed between 0.5 and 3 m/s may be achieved readily throughcontrol of acoustic energy above ejection threshold.

An alternative way to control characteristics of the droplet to reducethe likelihood of splashing or bounce is to control the Weber number ofthe impact. The Weber number is a dimensionless quantity intended toexpress the ratio of inertia force to surface tension force. It isdefined as ρV²l/σ where ρ is the density of the fluid, V is thevelocity, l is a characteristic length, and σ is surface tension. For adroplet impact l may be taken to be droplet diameter. Of theseparameters, it may be easier to control V and l. In the literature thereare indications that particular ranges of Weber numbers are associatedwith coalescence, at least with the fluids studied in the particularliterature references. Because of this, it may be desirable for theimprovement of coalescence and control of splashing and bounce tocontrol the parameters that make up the Weber number of a droplet inorder to confine that Weber number within particular ranges. Theparticular ranges may vary according to the composition of the dropletsand of the fluid into which the droplets are intended to merge. Theparameters that are controlled may include the velocity V and thecharacteristic length l. The characteristic length l may be controlled,for example, by using two smaller droplets in place of one larger one,if that would improve coalescence properties:

A further preferred way to control characteristics of the droplet is togenerate an electric field in a zone of space through which the dropletpasses. Preferably such an electric field is generated in the zone ofspace in which the droplet is generated.

An electric field in a zone through which the droplet passes may bedirected with different orientations. It is preferred that the electricfield be oriented along the direction of droplet separation from alarger mass of fluid.

An electric field in a zone through which the droplet passes may be ofdifferent magnitudes. For nanoliter scale droplets, it is preferred thatthe electric field lie between 1,000 and 100,000 V/m, preferably between10,000 and 100,000 V/m, more preferably between 25,000 and 50,000 V/m,and most preferably between 30,000 and 40,000 V/m. To the extent thefield is non-uniform, it is preferred that the field in the zone ofspace in which the droplet is generated lie within the preferentialranges just discussed. The desired intensity of the electric field wouldpreferably be increased relative to these values if the fluid of thedroplet is only modestly conductive, for example much less conductivethan pure water.

It is believed that an electric field component in the direction ofdroplet separation from a larger mass of fluid may cause the droplet tohave an electric charge when it is separated, and that the existence ofsuch electric charges can, if they are of appropriate magnitudes,facilitate droplet coalescence. The mechanism of facilitation is notclearly understood, but it is believed to be an effect above and beyondthe possible acceleration of the droplet by the electric field. Amechanism which has been hypothesized is an effective change in thedroplet's surface tension with the free charge present. The surfacetension is reduced because the charges on the drop surface repel oneanother.

With certain forms of droplet generation (e.g., acoustic ejection),droplet separation from a mass of fluid, is commonly preceded by theformation of a projection from the fluid surface. For a fluid of somedegree of conductivity, in the presence of an electric field, such aprojection should become charged so as to maintain it at an approximateelectric equipotential with the mass of fluid. It is hypothesized thatas the droplet forms from the projection, the projection is essentiallypinched off, and the net charge developed along the surface of theprojection is carried away with the drop. The charge on the droplet maybe approximately described by the relation Q˜4π∈₀ahE where a denotes thedroplet radius, h denotes the height of the mound at the time of dropletbreak-off, and E denotes the electric field intensity. In acousticejection, for DMSO/water mixtures the height h has been observed to beapproximately 5 times the drop diameter, so that h˜10a. An empiricalstudy of the charge on acoustically ejected droplets is presented aspart of Example 1 below.

It is believed that an electric field component in the direction ofdroplet arrival at the target may assist in the coalescence on accountof the polarization which the droplet will experience. In general, adielectric in the presence of an external electric field will develop adipole moment, corresponding to a separation of charge along thedirection of the external field. This redistribution of chargescounteracts the external field in such a way as to cause the netelectric field strength inside the dielectric to be smaller than theexternal field strength, and in the limiting case of a perfectlyconductive material, to be zero. In the particular case of a dropletapproaching and potentially coalescing with a mass of fluid, if there isan electric field in the direction of approach, the induced dipolemoment of the droplet will cause it to be attracted to the fluid mass.

Two limiting cases of this occur if the fluid mass is either (1) of thesame size as the approaching droplet, or (2) is much larger than theapproaching droplet, so that it can be considered an infinitehalf-space. These cases are very similar, especially for a relativelyconducting fluid, which in practice is common. For the case in which themass of fluid is very large, the induced dipole moment of the ejecteddroplet can be considered to generate a mirror dipole moment within thefluid mass, consistent with the method of images well known inelectrostatics, as depicted in FIG. 3. Treating the polarization of thedroplet as that of a spherical conductor (a reasonable approximation forhigh-dielectric-constant fluids), the dipole moment of the ejected dropmay be written as:p=4π∈₀ Ea ³where ∈₀ denotes the permittivity of free space, E denotes the appliedexternal electric field in the vicinity of the target, and a is theradius of the drop. For simplicity, we consider the geometry of FIG. 3,where z is in the (downward) vertical direction, and the target is takento be at z=0. The z-component of the electric field due to the “image”(upper) droplet, at the position of the arriving droplet, a height zbelow the fluid surface, can be written as:E _(z)=¼Ea ³ /z ³It follows that the force on the arriving droplet, due to theinteraction of its dipole moment with this electric field is:F _(z) =p dE _(z) /dz=3/2 π∈₀ E ² a ⁶ /z ⁴This force is directed upward, attracting the arriving droplet towardthe surface of the fluid at the target. The force, which varies as theinverse fourth power of the height of the droplet below the fluidsurface, will be strongest when the droplet is very near the surface ofthe receiving fluid. When the droplet just “touches” the receiving wellfluid, i.e. when z=a, the dipole force isF _(z)=3/2 π∈₀ E ² a ²

With an electric field of 30,000 V/m and 2.5 nl drops, the associateddrop radius is 84 μm and the dipole force with z=a is approximatelyF_(z)˜3×10⁻¹⁰ N. This may be compared to the gravitational force on thedrop mg˜2.6×10⁻⁸ N. Thus, the dipole force is only about one hundredththat of gravity.

For the other limiting case, in which the receiving mass of fluid is ofthe same size as the ejected droplet, the analysis is very similar tothat above. In this case, the receiving fluid mass forms a physicaldipole moment equivalent to that of the drop, in the presence of theexternal electric field. Thus, what was previously an image dipole at adistance 2z from the ejected drop, is now an equivalent dipole, at adistance z from the drop. Hence the dipole-dipole force between theejected drop and the receiving fluid mass is 8 times larger than thatfor the previous case, when the ejected drop is a distance z from thefluid mass. On the other hand, the limiting distance between the ejecteddrop and receiving fluid mass is now 2a. Thus the maximum force betweenthe ejected drop and receiving fluid mass is half that describedpreviously for the semi-infinite fluid mass. It is seen therefore thatthe polarization effects responsible for attraction of ejected drop tothe receiving fluid mass are of the same order of magnitude, for the twolimiting cases.

When using an electric field to reduce splash and bounce and/or improvedroplet coalescence, it may be desirable that the electric field notsubstantially affect the path followed by droplet prior to its arrivalat the target. It may even be desirable that the electric field notsubstantially alter the speed of the droplet, effecting for example achange of no more than 10% in the velocity. Such relative independenceof the path and speed from electric field may be helpful, for example,in order to make it easier to ensure that successive droplets aimed atthe same place on the target arrive at that place on the targetrepeatably. If the fluid on the target is itself being deposited dropletby droplet, as where the fluid transfer system is transferring a largerquantity of fluid as a series of droplets, such repeatability may bedesirable to facilitate coalescence. Studies in the literature such asthat authored by Professor Orme cited above indicate that the impactparameter has been seen as being of importance in coalescence ofraindrops. The independence of path and speed from electric field mayalso be helpful simply in order to make other parameters of the totalfluid transport system settable independently of the electric field.

A common way to generate an electric field in a region of space is tohave two electrodes A and B which are held at different potentials, suchthat the region of space overlaps with the region between theelectrodes. The electrodes may have a wide variety of shapes, forexample, flat sheets of solid sufficiently conducting material, materialof low conductivity coated or laminated with material of higherconductivity, rows of wires, and grids of wires. There is considerablefreedom in the placement of the electrodes, for example to avoidobstructing the flow of fluid or the movement of reservoirs and targets.Alternatively, as in examples 3 and 4 below, an electrode may be placedin the path of the fluid with holes to allow the fluid to pass through.

A voltage may be permanently wired into the system and the electrodesenergized at the fixed voltage whenever the system is powered on. Thiswould make sense, for example, if a particular electrode voltage isfound to be advantageous during generation of droplets in the operatingrange in which a fluid transfer system of the invention is expected tobe used. Alternatively, and for greater flexibility, it is possible tomake the voltages settable, for example, through the controller of thefluid transport system, or through one or more external inputs to thecircuit producing the electric field. Under certain circumstances it canbe advantageous to have a time varying voltage, for example a voltagewhich changes during different stages of the generation of each droplet,or a voltage which is held at a constant value for a predetermined time,for example during droplet generation, and zero otherwise.

As is discussed in detail in U.S. patent application Ser. No. 10/340,557to Mutz et al., various phenomena exist which can cause droplets to haveuncontrolled charge. It is possible to deal with uncontrolled charge invarious ways which involve in general terms providing a conducting path,or providing ionization, so that the fluid from which droplets areformed discharges. For example, the reservoirs in which the fluid isheld might be made from a sufficiently conducting material and grounded.

In connection with the embodiment of this invention in which the dropletcharacteristics are controlled by means of an electric field, it isfound to be helpful to also use measures for the reduction of electriccharge of the general type described in application Ser. No. 10/340,557.It is particularly helpful to have the reservoir holding fluid fromwhich droplets are made be an approximate or exact ground, usingtechniques discussed in that application such as ionization discharging.In that case, a single electrode held at a nonzero voltage with respectto ground can create the electric field which is used for dropletcontrol.

The voltages of the electrodes which are used to create the electricfield can be adjusted by those of skill in the art to achieve a desiredfield intensity. The adjustment can be, for example, purelyexperimental. It can alternatively be based on well known formulas(e.g., for the electric field between parallel plates) as found forexample in books on electromagnetics such as J. D. Jackson, ClassicalElectrodynamics (2d ed. 1975). It can alternatively be based on wellknown numerical techniques for computing electrostatic fields bynumerical solution of Laplace's equation.

Alternatively, the voltages of the electrodes may be adjusted in orderto optimize an experimental measure of splash, bounce, or coalescence.For example, one could run a number of ejections against suitabletargets and use a digital camera to capture images of the process ofdroplet impact on the fluid at the targets for human or automaticanalysis. Alternatively, one could use methods for scoring dropletcoalescence from visual examination of the target after impact, forexample by looking for multiple droplets where there should only be oneif coalescence occurred.

FIG. 4 depicts a particular embodiment of the circuit of the inventionwhich is preferred. The fluid transport system in which this embodimentis used is an acoustic ejection system with droplets ejected upwardsfrom a source reservoir to a target. In this embodiment the electrode isin the form of a grid, which is located behind the target. The sourcereservoir is maintained as an approximate ground by means of ionization,by the existence of true grounds in the vicinity, and/or by electricalconduction through the acoustic coupling medium.

As may be seen, the grid is driven by an N-type MOSFET 12 through a 5 MΩresistor 14. When the voltage on the gate of the N-type MOSFET is high(above threshold), the grid is connected to ground via the 5 MΩ resistor14. When the gate voltage is driven low, the grid is connected viaresistor 14 and another 5 MΩ resistor 16 to the output of a DC to DCconverter 18 which converts 0 to 5 V magnitudes to 0 to 1500 V. Thevoltage input of the DC to DC converter 18 is in turn connected to avariable voltage source 20 producing 0 to 5 V.

The gate of the N-type MOSFET 12 is connected via a 10 kΩ resistor 22 tothe output of the variable voltage source 20. The gate is also driven bythe output of a NAND gate 24 which allows two separate external controlinputs 26 and 28 to set the gate low and thus disconnect the grid fromground, connecting it instead to the output of the DC to DC converted18.

External logic (not shown) could employ the external inputs 26 and 28 toensure that when the fluid transport system is loading well plates orthe door is open, the grid is switched to ground via the N-type MOSFET12. When droplet ejection starts, the control inputs could be used tobring the grid up to a fixed voltage between 500 and 1500 volts DC, with800 V preferred. This voltage is applied evenly to the entire grid. Thiscreates a weak static field that extends from the target plate toapproximate ground planes in the drop ejection chamber and in thecoupling fluid under the source plate wells. After drop transfer, thegrid could be then switched back to ground. The grid is eithercompletely on, or completely off.

As will be understood by those of skill in the art, the components inFIG. 4 would also have power supply connections which, as is common inelectronics, are not explicitly shown.

In a variant on this preferred embodiment, the voltage alternatesbetween positive and negative levels in order to prevent charge buildupon the target wells. Variants are also possible where the voltage isturned on during well plate loading and where the grid is left floatingfor some portion of the ejection cycle.

In an alternative preferred embodiment, an ionizer which creates abiased ion cloud is used to charge target well plates as they enter thesystem. The biased ion cloud is created, for example, by driving theionizer with a suitably asymmetric or biased waveform. This ionizercould make use of standard ionization bars such as those made by JulieIndustries (Wilmington, Mass.). The target well plate is thenelectrically isolated during the ejection cycle. The electric fieldmodifying coalescence properties would be caused chiefly by the chargewhich was imparted to the target well plate by means of the biased ioncloud ionizer. Instead of imparting charge to the target well plate, itwould be possible to charge an object placed near the target well plateas an alternative way of generating an electric field.

In another alternative preferred embodiment, a corona discharge whichproduces ions of a particular polarity in a localized portion of spacemay be used to charge a droplet. Preferably a stream of gas directs theions produced by the corona discharge to the droplet. Alternatively, thedroplet may pass through the discharge. In this case, preferably thecorona discharge is stable, as for example between a pin and a plane.

Where an electric field is employed to improve droplet coalescence andthat electric field charges the droplets, the transfer of a large numberof such droplets into a single mass of fluid at the target may produce asignificant net charge in that mass of fluid, which would tend to screenan applied electric field and reduce its beneficial effects. One canestimate the field that might be produced by the deposition of thedroplet charge in the receiving mass of fluid. That charge would tend tospread across the receiving fluid meniscus as a surface charge sheet, sothat if the diameter of the fluid is d, the charge density of the sheetwould beσ=NQ/d ²where N is the number of droplets transferred and Q is the charge perindividual droplet. If we assume that the direction of droplet flightbetween source reservoir and target is the vertical z direction, thatthe field is produced by an electrode at voltage V above the target, andthat the source is an approximate ground, we get the equationE=σ/∈ ₀ +[V/z _(tf)−σ/∈₀]/[1+(∈₀/∈_(tf))(z _(elect) /z _(tf)−1)]Here z_(elect) denotes the height of the electrode above the sourcefluid, z_(tf) denotes the height of the target fluid meniscus above thesource fluid, and ∈_(tf) represents the permittivity of the targetfluid. For any aqueous target fluid, the quantity ∈₀/∈_(tf) would bemuch less than unity, and typically z_(elect)/z_(tf) would be somewhatgreater than unity. Thus, the quantity(∈₀/∈_(tf))(z_(elect)/z_(tf)−1)<<1, so that1/[1+(∈₀/∈_(tf))(z_(elect)/z_(tf)−1)]˜1−(∈₀/∈_(tf))(z_(elect)/z_(tf)−1),and the electric field may be approximated as:E˜(V/z _(tf))[1−(∈₀/∈_(tf))(z _(elect) /z _(tf)−1)]+(σ/∈_(tf))(z_(elect) /z _(tf)−1)

The first term of this expression,E_(init)=(V/z_(tf))[1−(∈₀/∈_(tf))(z_(elect)/z_(tf)−1)], is the fieldbetween source and target before any droplets arrive. The second term ofthis expression E_(drop)=(σ/∈_(tf))(z_(elect)/z_(tf)−1) is the portionof the electric field due to the deposited charge at the meniscus. It ispreferable, to avoid uncontrolled change in the actual electric field E,for E_(drop) to be considerably less than E_(init), for example tentimes less. Applying again the fact that(∈₀/∈_(tf))(z_(elect)/z_(tf)−1)<<1, we haveE _(drop) /E _(init)˜(σ/∈_(tf))(z _(elect) /z _(tf))/V

If Q=1×10⁻¹³ C, and d=3.5 mm, we have σ=8.16×10⁻⁹ C/m² per droplet. Ifz_(elect)−z_(tf)=5 mm, ∈_(tf)=80∈₀, and V=800 V, then the ratioE_(drop)/E_(init)˜(σ/∈_(tf))(z_(elect)−z_(tf))/V=76×10⁻⁶ per droplet.Thus, to keep this ratio below 1/10, it is desirable to deposit no morethan 1300 droplets. If it is desired to deposit more than that number ofdroplets, it might be worthwhile reversing the direction of the electricfield by reversing the polarity of V, so that charge of oppositepolarity starts to neutralize the accumulated a on the surface of thetarget fluid.

Variations of the present invention will be apparent to those ofordinary skill in the art. For example, the invention may be suitablefor use with any of the performance enhancing features associated withacoustic technologies such those described in U.S. patent applicationSer. No. 10/010,972, and Ser. No. 10/310,638, each entitled “AcousticAssessment of Fluids in a Plurality of Reservoirs,” filed Dec. 4, 2001and Dec. 4, 2002, respectively, inventors Mutz and Ellson and U.S.patent application Ser. No. 10/175,375, entitled “Acoustic Control ofthe Composition and/or Volume of Fluid in a Reservoir,” filed Jun. 18,2002, inventors Ellson and Mutz. In addition, the invention may be usedin a number of contexts such as handling pathogenic fluids (see U.S.patent application Ser. No. 10/199,907, entitled “Acoustic Radiation ofEjecting and Monitoring Pathogenic Fluids,” filed Jul. 18, 2002,inventors Mutz and Ellson) and manipulating cells and particles (seeU.S. Patent Application Publication Nos. 20020090720 and 20020094582).

It is to be understood that while the invention has been described inconjunction with the preferred specific embodiments thereof, that theforegoing description and the examples that follow are intended toillustrate and not limit the scope of the invention. Other aspects,advantages, and modifications within the scope of the invention will beapparent to those skilled in the art to which the invention pertains.

All patents, patent applications, and publications mentioned herein arehereby incorporated by reference in their entireties. However, where apatent, patent application, or publication containing expressdefinitions is incorporated by reference, those express definitionsshould be understood to apply to the incorporated patent, patentapplication, or publication in which they are found, and not to theremainder of the text of this application, in particular the claims ofthis application.

The following examples are put forth so as to provide those of ordinaryskill in the art with a complete disclosure and description of how toimplement the invention, and are not intended to limit the scope of whatthe inventors regard as their invention.

EXAMPLE 1

Within an acoustic ejection system, a conducting plate was situated 7 mmabove a 384 well plate in the source position (wells facing up). The 384well plate contained 30 μL of 70% DMSO/30% water in all wells. Theejection of 2.5 nL drops was then captured on video, with 0 V and with800 V applied to the upper plate. The acoustic coupling fluid, whichcontacts the bottom of the well plate, was held in contact with groundpotential. The electric field with the electrode at 800 V is estimatedto be 49,000 V/m based on a distance of 16.5 mm between the ground planeand the electrode. With electric field, the vertical position of the 2.5nL drops was Δz=61 μm higher at t=6.6 ms after ejection, compared tohaving the upper electrode grounded.

It is hypothesized that the difference of position caused by theelectric field was due to the droplet becoming charged during theejection. To calculate the charge q imparted to the droplet we proceedas follows.

The forces on a droplet of charge q at position z and time t are −gm forgravity, qE for electric field, and, if Stokes' law applies, −kv(t) fordrag where k=3πηn, η being the viscosity of air, 1.8×10⁻⁵ kg-s/m, and dbeing the droplet diameter. Applying Newton's law, we getm d ² z/dt ² =−gm+qE−k(dz/dt)which can be rewritten asm dv/dt=−gm+qE−kvwhere v=dz/dt is the velocity of the droplet in the z direction.

In general, a linear first order differential equation dv/dt+Av+B=0 withA≠0 has the solutionv(t)=(v ₀ +B/A)exp(−tA)−Bt/Awhere v(0)=v₀. Integrating;z(t)=(1/A)(v ₀ +B/A)(1−exp(−tA))−Bt/Aif z=0 at t=0. Here A=k/m and B=g−qE/m. The difference between z(t) withthe electric field E on and off is thus

$\begin{matrix}{{\Delta\; z} = {{( {1/A^{2}} )( {{qE}/m} )( {1 - {\exp( {- {tA}} )}} )} - {( {{qE}/{mA}} )t}}} \\{{= {( {1/A^{2}} )( {{qE}/m} )( {1 - {\exp( {- {tA}} )} - {tA}} )}},}\end{matrix}$so that q=Δz(m/E)(A²/(1−exp(−tA)−tA)). When A→0 (meaning the dragforce→0), taking limits in the formula for q, we get the simpler formulaq=2Δz(m/Et²).

The value m is about 2.7 μm given that the drop has a 2.5 nL volume and70% DMSO/30% water has a density of about 1.07. Values for E have beengiven above. The droplet diameter is ˜168 μm for a 2.5 nL sphere. Wethus have A˜10 s⁻¹ so that tA˜0.066 (dimensionless). Substituting thesevalues in the formula for q given above we get q˜1.6×10⁻¹³ C.

The difference in velocities with and without the electric field isgiven by Δv=qE/A(1−exp(−tA))˜0.02 m/s. As may be seen, in this examplethe electric field had little effect on the velocity of the droplet,which is on the order of 2 m/s.

EXAMPLE 2

A study was carried out with an acoustic ejection system to betterunderstand the influence of droplet speed on coalescence. A Krautkramer15 MHz F2 lens was used, and 70% DMSO/30% water was ejected into aninverted target well, filled with NaOH buffer. The receiving fluidsurface was slightly convex. Drops were ejected of nominal volume 5 nL,2.5 nL, and 1.25 nL.

First, the drop volumes and velocities were extracted from video data,as a function of power above threshold. These are shown in FIGS. 5Athrough 5C.

In these graphs, the displayed power is based on the amplitude deliveredto the acoustic transducer, which may be subject to saturation in theranges indicated. The data span the entire range from ejection threshold(0 dB always corresponds to threshold) to satellite threshold, so thatthe data should represent well the operating window for each ejectioncondition. The drop velocities quoted in FIGS. 5A-5C are the initialvelocities, directly above the source fluid.

FIGS. 6A-6C show curves representing the drop bounce probability (fromthe filled 384 well), as a function of the incident drop velocity at thedestination well. The velocities in these figures are velocities uponarrival at the destination well.

EXAMPLE 3

In a fluid transfer system based on acoustic ejection between wellplates and having an electrode behind the target plate, metal foil wasplaced over the source plate, and holes were punctured to access thewells. This metal aperture over the source plate was then grounded, andacted to screen the applied electric field from the source well fluid(the field strength with the foil present would be less than 1% of thatpresent without the foil). Thus there would be minimal droplet chargingwith the foil in place. With the foil present, the field between thesource plate and the destination well fluid would actually be increased,however, so that a dipole-dipole interaction would be enhanced. It wasfound that with the foil present, there was essentially no reduction indroplet bounce when the 800V was applied to the grid behind the targetplate.

EXAMPLE 4

A foil with apertures was used in the system of Example 3, but this timeover the target plate rather than over the source plate. The foil washeld at 800 V, as was the grid behind the target plate. With thisarrangement, there is essentially no electric field present within thetarget plate wells, but there is drop charging, due to the electricfield inside the source plate. It was found that for this scenario,there was excellent reduction in drop bounce.

We claim:
 1. A method for transporting one or more droplets of one ormore fluids from one or more reservoirs to contact one or more targets,the method comprising: generating acoustic radiation with an acousticpower level; ejecting a fluid droplet from a first fluid within areservoir by the generated acoustic radiation; transporting the ejectedfluid droplet at a droplet velocity towards a target; applying apredetermined electric field to the fluid droplet and the target,wherein applying the predetermined electric field comprises applying avoltage across first and second electrodes, the first electrodecomprising a grid disposed behind the target; generating a dipole withinthe fluid droplet by the applied predetermined electric field; andattracting the fluid droplet to contact the target by an interactionbetween the dipole and the target; wherein the applying a predeterminedelectric field to the fluid droplet and the target is performed withoutsubstantially affecting the droplet velocity; wherein the attracting thefluid droplet to contact the target includes increasing, by the appliedpredetermined electric field, a likelihood that the fluid droplet doesnot splash or bounce on contacting the target.
 2. The method of claim 1wherein the transporting the ejected fluid droplet at a droplet velocitytowards a target includes transporting the ejected fluid droplet upwardstowards the target.
 3. The method of claim 1 wherein the target is asecond fluid on a substrate.
 4. The method of claim 3 wherein theincreasing, by the applied predetermined electric field, a likelihoodthat the fluid droplet does not splash or bounce on contacting thetarget includes increasing, by the applied predetermined electric field,a likelihood that the fluid droplet coalesces with the second fluid onthe substrate.
 5. The method of claim 1 wherein the applying apredetermined electric field to the fluid droplet and the target affectsthe droplet velocity by no more than 10% in magnitude.
 6. The method ofclaim 5 wherein the applying a predetermined electric field to the fluiddroplet and the target affects the droplet velocity by about 1% inmagnitude.
 7. The method of claim 1 wherein a first direction of theapplied predetermined electric field and a second direction of thedroplet velocity are the same.
 8. The method of claim 1 wherein theapplied predetermined electric field is oriented along a direction ofthe droplet velocity.
 9. The method of claim 1, further comprisingcontrolling, with a circuit, a voltage across electrodes generating thepredetermined electric field such that the predetermined electric fieldsubstantially does not affect the droplet velocity and increases thelikelihood that the fluid droplet does not splash or bounce oncontacting the target.
 10. A method for transporting one or moredroplets of one or more fluids from one or more reservoirs to contactone or more targets, the method comprising: generating first acousticradiation with a first acoustic power level; ejecting a first fluiddroplet from a first fluid within a first reservoir by the generatedfirst acoustic radiation; transporting the ejected first fluid dropletat a first droplet velocity towards a target; applying a predeterminedfirst electric field to the first fluid droplet and the target, whereinapplying the predetermined first electric field comprises applying afirst voltage across first and second electrodes, the first electrodecomprising a grid disposed behind the target; generating a first dipolewithin the first fluid droplet by the applied predetermined firstelectric field; attracting the first fluid droplet to contact the targetby a first interaction between the first dipole and the target;generating second acoustic radiation with a second acoustic power level;ejecting a second fluid droplet from the first fluid within the firstreservoir or from a second fluid within a second reservoir by thegenerated second acoustic radiation; transporting the ejected secondfluid droplet at a second droplet velocity towards the target; applyinga predetermined second electric field to the second fluid droplet andthe target, wherein applying the predetermined second electric fieldcomprises applying a second voltage across the first and secondelectrodes; generating a second dipole within the second fluid dropletby the applied predetermined second electric field; attracting thesecond fluid droplet to contact the target by a second interactionbetween the second dipole and the target; wherein: the applying apredetermined first electric field to the first fluid droplet and thetarget is performed without substantially affecting the first dropletvelocity; and the attracting the first fluid droplet to contact thetarget includes increasing, by the applied predetermined first electricfield, a first likelihood that the first fluid droplet does not splashor bounce on contacting the target; wherein: the applying apredetermined second electric field to the second fluid droplet and thetarget is performed without substantially affecting the second dropletvelocity; and the attracting the second fluid droplet to contact thetarget includes increasing, by the applied predetermined second electricfield, a second likelihood that the second fluid droplet does not splashor bounce on contacting the target.
 11. The method of claim 10 wherein afirst direction of the applied predetermined first electric field and asecond direction of the applied predetermined second electric field areopposite to each other.
 12. The method of claim 11 wherein: the firstdirection of the applied predetermined first electric field and a thirddirection of the first droplet velocity are directly opposite to eachother; and the second direction of the applied predetermined secondelectric field and a fourth direction of the second droplet velocity arethe same.
 13. The method of claim 12 wherein the third direction of thefirst droplet velocity and the fourth direction of the second dropletvelocity are the same.
 14. The method of claim 11 wherein: the firstdirection of the applied predetermined first electric field and a thirddirection of the first droplet velocity are the same; and the seconddirection of the applied predetermined second electric field and afourth direction of the second droplet velocity are directly opposite toeach other.
 15. The method of claim 14 wherein the third direction ofthe first droplet velocity and the fourth direction of the seconddroplet velocity are the same.
 16. The method of claim 10 wherein thetransporting the ejected first fluid droplet at a first droplet velocitytowards a target includes transporting the ejected first fluid dropletupwards towards the target.
 17. The method of claim 10 wherein thetarget is a third fluid on a substrate.
 18. The method of claim 17wherein the increasing, by the applied predetermined first electricfield, a first likelihood that the first fluid droplet does not splashor bounce on contacting the target includes increasing, by the appliedpredetermined first electric field, a third likelihood that the firstfluid droplet coalesces with the third fluid on the substrate.
 19. Themethod of claim 17 wherein the increasing, by the applied predeterminedsecond electric field, a second likelihood that the second fluid dropletdoes not splash or bounce on contacting the target includes increasing,by the applied predetermined second electric field, a fourth likelihoodthat the second fluid droplet coalesces with the third fluid on thesubstrate.
 20. The method of claim 10 wherein the applying apredetermined first electric field to the first fluid droplet and thetarget affects the first droplet velocity by no more than 10% inmagnitude.
 21. The method of claim 20 wherein the applying apredetermined first electric field to the first fluid droplet and thetarget affects the first droplet velocity by about 1% in magnitude. 22.The method of claim 10 wherein: the first fluid and the second fluid arethe same.
 23. The method of claim 10 wherein: the first fluid and thesecond fluid are different.
 24. The method of claim 10, furthercomprising controlling, with a circuit, a voltage across electrodesgenerating the predetermined first electric field such that thepredetermined first electric field substantially does not affect thefirst droplet velocity and increases the likelihood that the first fluiddroplet does not splash or bounce on contacting the target and furthercomprising controlling, with the circuit, a voltage across theelectrodes generating the predetermined second electric field such thatthe predetermined second electric field substantially does not affectthe second droplet velocity and increases the likelihood that the secondfluid droplet does not splash or bounce on contacting the target.
 25. Amethod for transporting one or more droplets of one or more fluids fromone or more reservoirs to contact one or more targets, the methodcomprising: generating acoustic radiation with an acoustic power level;ejecting a fluid droplet from a first fluid within a reservoir by thegenerated acoustic radiation; transporting the ejected fluid droplet ata droplet velocity towards a target; applying a predetermined electricfield to the fluid droplet and the target; generating a dipole withinthe fluid droplet by the applied predetermined electric field; andattracting the fluid droplet to contact the target by an interactionbetween the dipole and the target; wherein the applying a predeterminedelectric field to the fluid droplet and the target is performed withoutsubstantially affecting the droplet velocity; and wherein the attractingthe fluid droplet to contact the target includes increasing, by theapplied predetermined electric field, a likelihood that the fluiddroplet does not splash or bounce on contacting the target; the methodfurther comprising reversing the applied predetermined electric field soas to neutralize accumulated charge at the target.
 26. A method fortransporting one or more droplets of one or more fluids from one or morereservoirs to contact one or more targets, the method comprising:generating first acoustic radiation with a first acoustic power level;ejecting a first fluid droplet from a first fluid within a firstreservoir by the generated first acoustic radiation; transporting theejected first fluid droplet at a first droplet velocity towards atarget; applying a predetermined first electric field to the first fluiddroplet and the target; generating a first dipole within the first fluiddroplet by the applied predetermined first electric field; attractingthe first fluid droplet to contact the target by a first interactionbetween the first dipole and the target; generating second acousticradiation with a second acoustic power level; ejecting a second fluiddroplet from the first fluid within the first reservoir or from a secondfluid within a second reservoir by the generated second acousticradiation; transporting the ejected second fluid droplet at a seconddroplet velocity towards the target; applying a predetermined secondelectric field to the second fluid droplet and the target; generating asecond dipole within the second fluid droplet by the appliedpredetermined second electric field; attracting the second fluid dropletto contact the target by a second interaction between the second dipoleand the target; wherein: the applying a predetermined first electricfield to the first fluid droplet and the target is performed withoutsubstantially affecting the first droplet velocity; and the attractingthe first fluid droplet to contact the target includes increasing, bythe applied predetermined first electric field, a first likelihood thatthe first fluid droplet does not splash or bounce on contacting thetarget; wherein: the applying a predetermined second electric field tothe second fluid droplet and the target is performed withoutsubstantially affecting the second droplet velocity; and the attractingthe second fluid droplet to contact the target includes increasing, bythe applied predetermined second electric field, a second likelihoodthat the second fluid droplet does not splash or bounce on contactingthe target; the method further comprising reversing the appliedpredetermined first electric field so as to neutralize accumulatedcharge at the target.
 27. The method of claim 26, further comprisingreversing the applied predetermined second electric field so as toneutralize accumulated charge at the target.